Gaseous discharge device



Jan. 8, 1963 Original Filed Oct. ll, 1955 w. ROBERTS 3,072,866

GAsEous DISCHARGE DEVICE 2 Sheets-Sheet 1 ATTORNEYS Jan. 8, 1963 l.. w. ROBERTS l 3,072,866

GASEOUS DISCHARGE DEVICE Crlgnal Filed Oct. 11, 1955 2 Sheets-Sheet 2 OLTAGE DISTANCE ACROSS GUIDE (Vo) DISTANCE ACROSS GUIDE Fig. 7

IN VEN TOR.

' LOUI W. ROBERT BY f ATTORNEYS Patented Jan. 8, i963 ire 3,072,366 GASEOUS DISCHARGE DEVICE Louis Wright Roberts, Roxbury, Mass., assigner to Microwave Associates, lne., Boston, Mass., a corporation of Massachusetts Original application Oct. 11, 1955, Ser. No. 539,785. Divided and this application .lune 2.6, 1959, Ser. No.

6 Claims. (Cl. S33- 13) this type result in an inherent limitation in the band widthl which the tube will encompasssince the resonant structure will be tuned to a specified center frequency. In addition, the tubes of this type have been limited in band width to some extent by the characteristics of the window utilized to seal the ionizable gas into the TR cavity.

lt is the object of this invention to provide an improved TR tube having a very wide band width and capable of handling large amounts of microwave energy. It is a further object of this invention to provide 'a TR tube having an improved keep-alive electrode structure.

It is a feature of this invention that it achieves wide band response characteristics through the use of a capacity loaded wave guide which produces the high potential gradient necessary for ionization breakdown without the necessity for resonant iris structures.

It is a further feature of this invention that it incorporates a keep-alive electrode structure which provides an ionization path of substantially constant and unvarying length together with an abrasion resistant electrode tip permitting long life.

The subject invention will be more easily understood by reference to the drawings in which:

FIG. l represents a plan view of the TR tube,

FIG. 2 represents a cross-sectional elevation along the tube axis,

v FIG. 3 is an end view of the tube,

FIG. 4 illustrates an enlarged detail View of the keepalive electrode fitted to its cavity,

FIG. 5 is a planview of one-half of the gu-ide showing `an alternative ridge configuration,

FIG. 6 is a plot of the voltage across a wave-guide section propagating the TEN mode, and

FIG. 7 is a plot showing the TR tube of this invention having opposed ridges providing a narrow gap.

' As illustrated by reference to FIGS. l, -2 and 3, this invention contemplates the utilization of a wave guide section it) having a rectangular cross-section and having flanges 12 at either end to facilitate attachment of the TR switch to other wave guide sections. Two pyramidalshaped ridges 14 and 16 project from the top and bottom surfaces of the rectangularA wave guide section. Each of these fins is composed of three parts, namely, a knife edge center ridge and two tapered ramp-like transition sections.` The two central longitudinally disposed knife edged ridges 18 are located adjacent the axis of the tube. These knife edges, in opposed relationship on the top and bottom faces of the tube, provide a narrow gap 2i) across the axis of the tube. A high potential gradient will exist across this gap because of the difference in potential between the upper and lower wave-guide surfaces, and it is this potential gradient which will, upon receipt of a high power pulse by the tube, cause the ionizing breakdown of the gas in the tube thereby effectively short-circuiting the transmission of microwave energy.

On each end of the knife edged portion of each ridge there extends a tapering impedance matching ramp-like section i9 extending from the widow at either end of the tube to the knife edged ridge.

The ends of the wave guide section liti are sealed by the use of a wide band window 2d preferably in the form of multiple iris windows having three openings of substantially the same length, 26, 2d and liti in FG. 3, the central opening being substantially wider than the two outer openings. Multiple iris windows for wide band perform-ance have been described in the literature by Reingold, Carter and Garotf of the Signal Corps Engineering Laboratories in a report entitled Single and Multi-iris Resonant Structures, published in the proceedings of the iRE. However, the multiple iris windows therein described were of uniform width. The window utilized in this TR tube represents a substantial improvement in uniformity of performance over a wide band width. For example in the 2 centimeter to 3 centimeter wavelength region, when the middle slot is widened to .148 inch (by .7 inch long) and the two outer slots are maintained at .094 inch, the insertion loss of these windows is approximately .03 db over a band width of 4,000 mc./s. centered .at 10,@30 mc./s. lt is obvious that a wideband window is essential to a successful tube since the band width of the system will be limited to that of the poorest link.

As in any TR tube, it is desirable to provide a keepalive electrode structure which will furnish a residual supply of electrons adjacent the discharge gap. In the structure of this invention two such electrodes 32 and 35i are provided along the length of the ridge i8. Since the entire pyramidal ridge is a very wide band device which does not use resonant structures, the spacing of the eleceffective depending upon the frequency. However, both electrodes are connected at all times in order to maintain effective keep-alive action over the entire range of frequencies utilized by the tube.

As illustrated by reference to the enlarged detail view in FIG. 4, the keep-alive electrode is composed of a conductive rod 36 surrounded by an insulating dielectric 38 fitted to a conical chamber dit in the tube section itl, said chamber having an outer metallic wall li-Z. it has been found that greatly improved performance, both in this TR tube and in TR tubes of conventional design, may be obtained by inserting adjacent the conical wall i2 a preformed insulating cone i4 of dielectric material. This dielectric insert has the effect of restricting the ionizing gap to a single path length running from the tip 46 of the rod 36 to the bottom edge 48 of the conical wall 42. This end is adjacent the gap 2i? in the TR tube structure. The parts of the electrode are sealed to a metal sleeve 5d by means of a glass seal 52 and the sleeve in turn is soldered to the body l0.

A tip 46 of the electrode rod 36 is formed of a separate hardened pellet fused to the end of the electrode 36. This tip structure may be of stainless steel or of titanium dioxide reduced in hydrogen to give a content of 10% metallic titanium. In either event, the function of this hardened tip is to resist ion bombardment which in the case of the ordinary keep-alive electrode tends to wear the electrode away and change the over-all gap dimensions as well as the uniformity of the gap.

In the conventional TR tubes, a variety of paths are presented between the electrode tip 46 and the wall 42. The variation in discharge path has resulted in non-uniform tube action. Furthermore, insuiating the inner surface of the wall 42 by means of a conventional baked-on insulating sheath is rendered diiiicult or impossible by the tendency for any such ceramic coating to withdraw from the end 48 of the wall 42 or to be perforated by the pin holes common to baked ceramic insulation. The insert 44 is preformed to extend over the entire inner surface of the cone thereby achieving greatly improved uniformity of keep-alive action.

For rectangular wave guides having a width to height ratio of 2.25 to l, it has been experimentally determined that the broad dimension of the guide should closely approximate l.8 times the width of the ridge base in the central knife edge section. if the ridge base width does not approximate .56 times the guide width severe spurious mode problems are created. If the ridge is too wide there is a tendency to create unwanted modes such as the T1521, T543, and 'H565 modes. if the ridge is too narrow the mode problems are accompanied by excessive leakage power.

In the preferred and illustrated embodiment, the width of the ridge base is uniform from window to window. However, some further reduction in leakage power is possible at the expense of greatly increased problems of fabrication by increasing the width of the ramp bases from .56 times the wave guide Width at the central knife edge section to the full width of the guide adjacent the windows. A plan View of this ridge configuration is shown in FIG. 5. The elevation view of such a ridge would be the same as that illustrated by reference to FIG. 2. The reference numbers are identical to those in FIG. l for like parts except that they are primed.

The preferred embodiment incorporating a ridge of uniform width at the base covers a range of from 2.4 to 3.6 centimeters and exhibited standing wave ratios not in excess of 1.1 over the entire range. The tube incorporates a .006 inch gap between knife-edge ridges .4 inch long. The tapered ridge sections are 1.3 inches long giving a compact overall dimension of 3 inches. The tube is capable of handling power levels of up to 100 kilowatts peak over this range without excessive leakage.

The ridge configuration is in fact capable of satisfactory operation over several times the above band width up to approximately centimeters. However, the length of the ridge is not critical with respect to a specific center wavelength since the tube operates over a wide band width. The length of the waves in the region of the ridge in fact differs markedly from free space or open wave guide propagation and these wave lengths become, in fact, irrelevant with regard to ridge dimensions. It has been experimentally discovered, however, that over the 8000 mcgacycles to 12,400 megacycles range the ridge need not be longer than the above .4 inch and that the standing wave ratios tend to increase if the ridge is as long as 2 inches. Because of the ridge ditortions this .4 inch length is of the order of Mt wave length in the band width of interest and in any event, it may be said that the ridge length is preferably less than the shortest wavelength of interest.

The effect of the ridge configuration on the tube operation may be shown by reference to FIGS. 6 and 7. In FIG. 6, the voltage intensity across an ordinary rectangular guide propagating the TEU, mode is shown. It may be shown that this intensity is a function of the cosine of the distance across the guide. The effect of the opposed ridges is to concentrate the field intensity in the region of the ridges, and the resulting plot of field intensity is shown schematically in FIG. 7. The intensity may be shown .to be a function of the cosine to some power n where n is greater than 2. The intensity of the electric field in the region of the gap is therefore very high and is in fact more than sufficient to produce an ionizing breakdown upon receipt of a high power pulse.

While this invention has been described with respect to a single embodiment, it will be apparent that the same design principles may be applied at other frequencies, and that the invention may be modified in a variety of ways without departing from the scope of this invention which is encompassed in the following claims. This application is a division of my copending application Serial No. 539,785, filed October 11, 1955.

I claim: i

1. ln a gaseous discharge device having means for providing a high voltage gradient across a gap, an ionizing electrode structure adjacent said gap comprising an electrically conductive wall defining a cavity having an edge adjacent the gap, an insulated electrode located along the axis of the cavity and having an end positioned a predetermined distance from the cavity edge, and a preformed insert in the form of an additional insulating sheath of solid dielectric material fitted to the cavity wall and extending substantially to said edge to restrict the ionizing path to the distance between the cavity edge and said end of said electrode.

2. In a gaseous discharge device having means for providing a high voltage gradient across a gap, an ionizing electrode structure comprising an electrically conductive wail defining a cavity having an edge adjacent the gap, an electrode rod located along the axis of the cavity, first insulating means for the electrode rod, an additional insulating sheath of solid dielectric material for the conductive cavity wall extending to but not covering the cavity edge and a hardened electrode tip at the end of said rod, the end of said tip having a predetermined displacement from the cavity edge.

3. In a gaseous discharge device having means for providing a high voltage gradient across a gap, an ionizing electrode structure adjacent said gap comprising an electrically conductive wall defining a cavity having an edge adjacent the gap, an insulated electrode comprised of an elongated conductor surrounded by a first electrically nsulating sheath clad to the sides thereof, an end of said conductor extending free of said sheath, said electrode extending along the axis of the cavity and having said end positioned a predetermined distance from the cavity edge, and a preformed insert in the form of a second electrically insulating sheath of solid dielectric material tted to the cavity wall and extending substantially to said edge and spaced from said first sheath to restrict the ionizing path to the distance bet 'een the cavity edge and said end of said electrode.

4. In a gaseous discharge device having means for providing a high voltage gradient across a gap, an ionizing electrode structure comprising an electrically conductive wall defining a cavity having an edge adjacent said gap, an electrode rod located along the axis of the cavity, first insulating means clad to the side surface of said rod but not covering the end portion thereof, said end portion positioned a predetermined distance from the cavity edge, said cavity being tapered toward said edge, and second insulating means of solid dielectric material in the form of a tapered preformed hollow insert adjacent the cavity wall and spaced from said electrode rod, said insert extending to but not covering the cavity edge, to restrict the ionizing path to the distance between said cavity edge and said end portion of said electrode rod.

5. In a gaseous discharge device having means for providing a high voltage gradient across a gap, an ionizing electrode structure adjacent said gap comprising an electrically conductive wall defining a cavity having an edge adjacent the gap, an ionizing electrode located along the axis of said cavity and having an end positioned a predetermined distance from the cavity edge, and insulating means of solid dielectric material spaced from said electrode and fitted to said Wall and extending substantially to said edge to restrict the ionizing path to the distance between the cavity edge and said end.

6. in a gaseous discharge device having means for providing a high Voltage gradient across a gap, an ionizing electrode structure adjacent said gap comprising an electrically conductive wall defining a cavity tapering in cross section toward said gap and having an edge adjacent the gap, an ionizing electrode located along the axis of said cavity and having an end positioned a predetermined distance from the cavity edge, and insulating means of solid dielectric material spaced from said electrode and iitted 6 to said wall and extending substantially to said edge to restrict the ionizing path to the distance between the cavity edge and said end.

References Cited in the tile of this patent UNITED STATES PATENTS 2,631,255 Stavro Mar. 10, 1953 2,697,800 Roberts Dec. 21, 1954 2,748,351 Varnerin May 29, 1956 2,819,422 Gates Jan. 7, 1958 2,830,231 Scott Apr. 8, 1958 2,918,603 Alexander Dec. 22, 1959 

1. IN A GASEOUS DISCHARGE DEVICE HAVING MEANS FOR PROVIDING A HIGH VOLTAGE GRADIENT ACROSS A GAP, AN IONIZING ELECTRODE STRUCTURE ADJACENT SAID GAP COMPRISING AN ELECTRICALLY CONDUCTIVE WALL DEFINING A CAVITY HAVING AN EDGE ADJACENT THE GAP, AN INSULATED ELECTRODE LOCATED ALONG THE AXIS OF THE CAVITY AND HAVING AN END POSITIONED A PREDETERMINED DISTANCE FROM THE CAVITY EDGE, AND A PREFORMED INSERT IN THE FORM OF AN ADDITIONAL INSULATING SHEATH OF SOLID DIELECTRIC MATERIAL FITTED TO THE CAVITY WALL AND EXTENDING SUBSTANTIALLY TO SAID EDGE TO RESTRICT THE IONIZING PATH TO THE DISTANCE BETWEEN THE CAVITY EDGE AND SAID END OF SAID ELECTRODE. 